The present invention relates to a xerographic printing apparatus, and in particular embodiments relates to a system and method for extending the useful life of a non-contact charge receptor, such as a photoreceptor, and extending the useful life of a customer replaceable unit used in the printing apparatus.
Electrostatographic printing methods, such as xerography, involve creation of an electrostatic latent image on a charge receptor, such as a photoreceptor (PR). As is well known, in such apparatus, the photoreceptor is imagewise discharged in a manner conforming to an image desired to be copied or printed, and then this latent image is developed with toner. The developed toner image is in turn transferred to a print sheet, which is then fused to fix the transferred toner image thereon.
Charging involves contact charging of a photoreceptor by a bias charge roll (BCR). Its main advantage is its low footprint. Thus it is particularly suited for charging small diameter organic photoreceptor drums (OPC) drums used in low and mid-volume black and white (B/W) and color machines. Conventional BCR charging is based on a DC-offset AC excitation waveform. As a result a stable photoreceptor charge voltage V-hi controlled by the DC bias is achieved when Vpp, the AC peak to peak voltage, is greater than a threshold voltage, V-th. Print quality considerations such as background disappearance and halftone uniformity require Vpp and IAC somewhat greater than the threshold values. Moreover, the trend toward increasing process speed in OPC drum based machines, particularly in tandem color applications, leads to even higher AC current requirements.
A drawback of contact AC BCR charging is the significant limitation it imposes on PR life because degradative AC corona species are generated in close proximity to the PR surface. Approaches to extend PR life include the development of hard PR overcoats and corona resistant CTL materials (e.g., PTFE filled CTLs) as well as a variety of excitation waveforms such as DC, clipped AC or pulsed bias waveforms, each with varying degrees of success. These approaches have many limitations. For example, PR degradation and wear associated with corona discharge limits the useful life of the customer replaceable unit and drives up run costs for the xerographic printing apparatus. Thus, the proximity of an AC corona discharge at the photoreceptor surface is associated with an increase in the total cost of ownership of the xerographic printing apparatus.
DC BCR charging is a very effective means of improving wear life, but BCR sensitivity to contamination by toner and PR degradation products generally precludes its practical use. Pulsed bias and clipped AC excitation waveforms have been shown to greatly improve PR wear life but a stable V-hi cannot be attained with the latter. Instead V-hi increases monotonically as V-pp and IAC increases. Thus, practical implementation of pulsed biased and clipped AC excitation waveforms would require complex controls to achieve V-hi stability especially across environmental conditions, and may be difficult to achieve.
The properties of the charge receptor, such as a photoreceptor, affect the overall functioning of a printing apparatus and the ultimate quality of images created therewith. The electrical stresses placed on a photoreceptor by printing of thousands of images contributes to the degradation of the photoreceptor. As the photoreceptor degrades the quality of images that can be created by that photoreceptor degrades as well. Thus, in practical embodiments of xerographic printers and copiers, it is inevitable that the photoreceptor will be periodically replaced. Replacement of the photoreceptor represents a large expense. It is therefore desirable to provide a method and system by which the photoreceptor, even a pre-existing photoreceptor, can be extended significantly.
Other factors that shorten the life of the bias charge rolls include toner and additive contamination. Such contamination is another cause of non-uniform charging of the BCR. Further, various methods of cleaning the BCR fail to sufficiently solve these problems. For example, contaminants in cleaners are densely impacted on the BCR surface due to the BCR loading force against the PR. These densely packed contaminants are often not removed by typical cleaning methods. Thus, even after cleaning, the effect of non-uniform charging of the BCR remains.
As BCR technology advances to include faster color machines, it is not uncommon to use higher AC currents. The use of higher AC currents in these applications typically causes an even higher wear rate of the photoreceptor. Thus, the demands of advancing technology expose even greater shortfalls associated with customer expectations regarding the usable life of a customer replaceable unit.
U.S. Pat. Nos. 5,543,900 and 5,613,173 disclose a type of charging apparatus for use in charging the photoreceptor in a xerographic printer. In combination with the bias roll which initially charges the photoreceptor a special “clipping” circuit is disclosed comprising a diode and resistor. The clipping circuit has the function of clipping an oscillating voltage applied to the bias roll, and in turn to the photoreceptor, as the bias roll charges the photoreceptor. The long-term effect of this clipping is that lower electrical stresses are experienced by the photoreceptor with extended use, and in turn the degradation of the photoreceptor is inhibited.